This invention relates to an apparatus wherein solids which have formed a sediment in liquids are continuously withdrawn from a processing chamber into a collecting chamber which is also filled with fluid. The solids can be, for example, ion exchangers, catalysts, active carbon or the like. More particularly, the present invention relates to such an apparatus to discharge a mixture of liquid and solid into the collecting chamber from where the liquid is supplied to a pump, and the solid is discharged for subsequent supply to a reactivation unit after which it is returned to the processing chamber.
As is known, in prior art withdrawal devices the solids are conveyed from a processing chamber to an underlying second chamber. Most such withdrawal devices are operatively controlled on the principle of employing a movable slide or valve. A drive mechanism, located externally of the chamber, is mechanically connected to the movable slide or valve. In other known forms of withdrawal devices, the solid is entrained by a jet of liquid and the device operates on an injector principle. Such known devices have a number of disadvantages which are summarized by the following. When there is a large throughput of solids in the processing chamber, the efficiency of the process often depends on the losses of solid material. In a typical example, losses of solids affects the efficiency of continuously operating active carbon processes used for removing organic substances from waste water. The cost of active carbon is relatively high and in view of its relatively low mechanical strength, the active carbon must be treated and handled in a relatively gentle manner. Most of the losses result from the mechanical destruction of particles of active carbon. It is therefore very important to insure that, in the region of the moving liquid phase, the liquid to carbon ratio is sufficiently great to insure that the particles of carbon do not rub against one another or against other parts of the apparatus at high speeds. The active carbon in the liquid phase, i.e., the mixture of active carbon and liquid, has a small angle of repose, usually below 15.degree.. Therefore, in known apparatus, valve seats, slides or closure plates, must be located near the outlet pipe which must have a minimum diameter due to the self-blocking effect of the active carbon. Furthermore, the active carbon which is discharged must flow from these devices when there is a low fluid to carbon ratio. This inevitably results in prolonged contact on the particles with one another and with the closure element. When the valve or slide member closes, particles of carbon are caught between the closure elements and the outlet pipe. Mechanically driven closure elements are subject to natural wear and such elements must be made of special materials when the fluids are corrosive. To insure reliable operation, the entire device requires continual maintenance.
In the case of continuously operating plants, it is usually necessary to determine the amount of solids in circulation. Experience has shown that, in the case of known withdrawal devices, it is impossible to plot or otherwise prepare a simple curve from which, for example, the amounts of discharged solids can be read off as a function of the valve or slide plate position. Measuring devices required for this purpose are separate, external, and very expensive.